Employee wellness firms and insurers are working with companies to mine data about the prescription drugs workers use, how they shop and even whether they vote, to predict their individual health needs and recommend treatments.

Trying to stem rising health-care costs, some companies, including retailer Wal-Mart Stores Inc., are paying firms like Castlight Healthcare Inc. to collect and crunch employee data to identify, for example, which workers are at risk for diabetes, and target them with personalized messages nudging them toward a doctor or services such as weight-loss programs.

Companies say the goal is to get employees to improve their own health as a way to cut corporate health-care bills.

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Privacy advocates have raised concerns about such practices. Employees generally have a choice in whether to participate in the programs. The services are new enough that relatively few workers are aware of them.

“I bet I could better predict your risk of a heart attack by where you shop and where you eat than by your genome,” says Harry Greenspun, director of Deloitte LLP’s Center for Health Solutions, a research arm of the consulting firm’s health-care practice.